Multiple transconductance vacuumtube amplifier



Patented Mar. 25, 1941 UNITED f STATES PATENT oer-"ice Robert L. Freeman, Flushing, N. Y., assignor to Hazeltine Corporation, a corporation of'Delaware ' Application October 25, 1939, sci-n1 No. 361,141

1 Claims. .(Cl. 179-171) This invention relates to vacuum-tube amplitiers and more particularly to such amplifiers wherein the nature of operation is such as to provide a negative transconductance in addition 5 to the usual positive transconductance.

As used herein the term transconductance" is definedas the rate of the change of electron current to a first electrode of a vacuum tube with respect to the voltage of a second electrode there- 14) of when all electrodes, excepting said second electrode, are maintained at constant potentials. When a positive increase in voltage of the secondelectrode is accompanied by. an increase in I electron current to the first electrode, the transconductance is positive; the transconductance is "negative" whenadecrease or negative increase involtage of the second electrode is accompanied by an increase in electron current to the first electrode.

20 Heretoiore, attempts to increase the amplification of vacuum-tube amplifiers have been limited by circuit factors, such as the power factor of associated windings, and by the characteristics of vacuum tubes, such as the influence of tube self-conductances on the associated circuits,

which, in turn, restrict the maximum ratio of the reactance of the circuit to the eiiective resistance thereof and, thus, the gain of the amplifieiz 30 Conventional amplifiers oi the prior art generally utilize only the positive transconductance between the c'ontrol electrode and anode of a vacuum tube. Also, in the prior art, the dynatron" or negative resistance characteristic of 35 screen-grid tubes is well known. In the operation of a dynatron, when there is appreciable secondary emission at the anode, there is a relatively restricted region where the anode current increases as the anode voltage is reduced, which 40 represents a negative resistance characteristic.

Owing to the unstable, limited, and nonuniform nature of the operation of such tubes, however, attempts to utilize the dynatron as an amplifier have been generally unsatisfactory.

43 It also has been proposed touse a vacuum tube of the hexode type as an amplifier wherein the electrodes are connected to provide a triode section and a tetrode section in series relation or cascade, but, while highv amplification theoreti- 9 cally is obtainable in this way, practical dimculties occur, for example, anode-control electrade feedback in the triode section which tends to cause oscillation. However, since such. proposal has not otherwise gone beyond operating 55 the vacuum tube in a normal manner, insofar and a second anode.

as using only positive transconductances are concerned, it has met with little favor and has remained only a theoretical possibility.

It-is an object of the invention, therefore, to provide an improved, simple and economical 5 vacuum-tube amplifier which overcomes the limitations heretofore encountered in amplifier design and thereby makes possible higher amplification and, if desired, greater selectivity than Eves economically possible prior to the inven- 10 ion.

It is a further object of the invention taprovide acascade vacuum-tube amplifier comprising a single vacuum tube wherein a negative transconductance is utilized.

It is a still further object of the invention to provide a vacuum-tube amplifier comprising a vacuum tube having an effective negative anode resistance, which is stable in operation and which may be reproduced with uniformity.

It is another object of the invention to provide an improved push-pull amplifier havingan exceptionally high amplification.

In accordance. with the invention, there is pro vided an amplifier comprising a vacuum tube having at least a cathode, a first control electrode, a first anode, a second control electrode,

An input circuit is coupled to the cathode and first control electrode.

A first output circuit is coupled to the cathode and first anode through a two-terminal imped-' ance network having a substantially uniform resistance over a desired frequency band, while an anode or a second output circuit is coupled to the cathode'and second anode, the second output circuit including a load impedance of any suitable nature. The first anode and second control electrode are so coupled together and the vacuum-tube design andoperation are such that a negative transconductance exists between said 4 second control electrode and first anode, whereby a negative resistance is reflected across the first output circuit which tends to cancel the resistance thereof and the dynamic first anode-cathode self-resistance. The. two-terminal network preferably includes the interelectrode capacitance between the cathode and first anode as well as the interelectrode capacitance between the cathode and second control electrode.

In one form of the invention, the arrangement of the amplifier circuit is such that, by an optimum choice of the value of the uniform resistance of the two-terminal network, together with suitable values of both negative and positive transconductances oi! the vacuum tube, the sig- 5:

is in phase with the signal applied to the input circuit and 180 degrees out of phase with the sigrial-output voltage of the first output circuit. Between the first control electrode and first anode, aswell .asbetween the second control electrode and second anode, there preferably are provided screen-grid means to inhibit energy transfer resulting in undesired feedback in the amplifier.

In another aspect of the inventiomaregenerative amplifier is provided comprising a vacuum tube having a cathode, two control electrodes and two anodes; an anode circuit coupled to said cathode and one of said anodes'iarthest .iroin the cathode, an input circuit coupled to the oathode and the control electrode nearest to'the cathode; and an outputcircuit coupled to the oathode and the other anode through a two-terminal network having a uniform resistance over a desired frequency band; together with ieed-hack means coupling the other control electrode to said other anode whereby regeneration of the amplifier is obtained by virtue of a negative transconductance between said last-named electrodes.

For a better understanding of the present invention, together with other and further objects thereof, reference is bad to the following description taken in connection with the accompanying drawing, and its scope will he pointed out in the appended claims.

Referring to the drawing, Figs. 1 and 2 are circuit diagrams of two embodiments of the vacuum-tube amplifier of the invention; Figs. 1a and 2a are electrical equivalent circuits in a form suitable for analysis; and Fig; 3 is a schematic A cross-sectional view of a preferred form of vacuum tube.

Referring to Fig. 1 of the drawing, there is shown an amplifier comprising a, vacuum tube I0 having a cathode li, a first control electrode 12, a first anode l3, a screen grid M, which may be omitted in certain low-frequency applications, a second control electrode i5, and a second anode !6. An input circuit having terminals ll, I! is coupled to cathode II and control electrode 02 through a source of bias voltage, such as a bat- .tery 58, An output circuit having output terminals I 9', i 9' is coupled to anode l3 and cathode H through a two-terminal impedance network is having a substantially uniform resistance 'over a desired frequency band, this network comprising in its simplest form a-reslstance shunted by the tube and stray circuit capacitances. The first anode I3, the second anode l6, and screen M are-supplied with suitable operating potentials from sources indicated as +31 and l-Ba and -+Sc, respectively, whileathe second control electrode I5 is connected through a high impedance, such as a resistor 20, to a source of negative-bias potential indicated as C. Thus, an anode circuit is coupled to the second anode l6 and cathode ll which includes means whereby a positive potential is applied to the anode IS. The second control electrode I5 and the first anode 13 are coupled by means including a condenser 2|; By-

pass condensers 22 are provided in the various circuits where needed to by-pass currents at signal frequencies to a common terminal, such as ground.

The design and operationof the vacuum .tube' iii are such as to provide a negative transconductance between the second control electrode IS- and the first anode. l3; thatis, the tube configuration and choice of operating potentials should be such as to produce suflicient electronic space charge between'the positively-biased first anode i3 and the succeeding negatively-biased second control electrode 85 so as to reduce the potential to zero in this space charge region. In the art such space charge condition, is known as a virtual cathode. .eis is well known, there is a negative transconductance from an electrode following a virtual cathode in the. tube space path to an electrode preceding it which in this case may be termed 015, 1a.

A variation of the potential only 01' the first anode i3 causes an in-phase variation of the electron current flowing to this said electrode; thus the dynamic self-resistance between the first anode-cathode is a positive resistance of value designated as l/gia. On the other hand, since gm, i3 is generally considerably greater than m a like variation of the potentials of both electrodes i3 and it causes the variations in current to the first anode i3 to be 180 degrees out of phase with said potential variations. Thus, if the potential variation applied to electrodes l3 and i5 is a periodic voltage designated as E2, the current I13 flowing to the first anode i3 is I1s=E2yiaE2yi5, 13=E2(g13715, 13) (1) wherein the signs of 9'13 and 915,13 have been taken care of in the equation and thus only absolute values are to be assigned to these quantities.

In Fig. 1 it is seen that the signal output potential Ea developed across network is is eifectively applied to both electrodes I3 and I8 ,due to their coupling through condenser 28. Conseuuently, the circuit arrangement cooperating with the tube presents an effective resistance between the first anode l3 and cathode H which is:

E 1 I18 gi3 gi6,l8 (10) Since 015, 1a is normally much greater than an, we may express this efi'ective resistance as -l/gis,1a. Thus, it is seen that this negative eflective resistance is reflected in shunt with network l9 and-tends to cancel the actual positive resistance thereof as well as the dynamic selfresistance of the first anode-cathode circuit. That is to say, the amplifier tube It has an effective negative first anode resistance. This is useful for obtaining high gain where the output impedance is a uniform pure resistance over the pass band, or for increasing'the ratio Q of reactance' to resistanceof a tuned output circuit which also determines the gain of the system. The arrange,- ment of the second control electrode I! with relation to the first anode IS, the coupling of these electrodes through condenser 2|, and the source of negative-bias potential C thus comprise means providing a negative transcon'ductance between the second control electrode I5 and the first anode I! for reflecting a negative resistance acrom the output circuit of the anode ll which tends to cancel the actual resistance thereof and the dynamic first anode-cathode self-resistance. The impedance network l9 and the condenser 2| comprise means for deriving a potential from the first anode l3 and for applying it without substantial phase shift and with the same polarity (to the second control electrode i5.

Referring to Fig. 1a, which is electrically may be analyzed as f lows: The gain amplifier is:

of the E-; me

but

Is -d112, iaE1+g15, 1aE2-qi3Ez or. since i "gm, lZl l+gl5. is:

gi2.1n i I 915. 13k so that "gm. 12R 1'11 1-g15 l In these equations the first subscript of the transconductance symbol g refers to the particular control electrode with respect to which transconductance is measured while the second refers I to the output electrode whose current is affected by variations of potential on the control electrode. The other symbols represent the quantities indicated in Fig. 1a except the factor R, which is the uniform resistance presented by output network l9 over the frequency band. The transconductance 715, 13 is negative, but the sign has been taken care of in Equation 3 so that absolute values are to be inserted in Equation 5.

Now Equation may be viewed as the gain equation for a conventional amplifier whose gain is g12, 13Rcff where Ref! is the effective resistance of the parellel combination of resistance R and 1/g15, 13. Thus, the amplifier, when viewed at a conventional amplifier, has a negative anode resistance of a value equal to 1/g15,13. The ain becomes very large as Q15,13R approaches unity. Ifthis product equals or exceeds unity, unwanted oscillations will result.

Successful operation of the amplifier requires that the circuit conditions be such that a virtual cathode, as pointed out above in detail, is formed between anode l3 and control electrode l5 in order to obtain a negative transconductance therebetween. Under conventional triode operation of a tube of the configuration of electrodes ii, l2, and I3, the gain 012,13 would be small. since in this region electrons fiow in one direction onlyand those intercepted by the anode l3 are proportional to the ratio of its intercepting area to the cross-sectional area of the electron path. When a virtual cathode is present, however, most of the electrons returned from the virtual cathode are collected by the anode 13, thus making m2, is large.

In simplest form the output network 19 consists of a resistance R shunted by unavoidable interelectrode and stray circuit capacitance. An expression for gain in this case is obtained in the same manner as Equation 5 by assuming the output admittance to consist of a conductance l/R.

and a susceptance a'wC. Then ventional amplifier having the same load admittance which gain is given by the expression:

-2 Atan 1 which is approximately uniform over the pass band. To adapt the circuit of Fig. 1a to broadband usage, the two-terminal impedance network 19 is used. This network must include the tube capacitance as an element and provide an approximately uniform resistance over the pass band.

Referring to Fig. 2, there is shown a single tube cascade amplifier embodying the amplifier of Fig. 1 as the first stage thereof, similar elements having-identical reference numerals. A load 23, comprising an impedance Z, is included in the second output circuit of the amplifier, while a screen grid 24 is disposed between control electrode l5 and the anode IS in addition to the screen grid I4 between the anode l3 and control grid 12. Regeneration. is obtained by feed-back means comprising the condenser 2| coupling the control electrode 15, acting as a control electrode of a first stage of amplification, directly to the first anode l3 of the same stage, as in the arrangement of Fig. 1. Thus, as further pointed out below in connection with the analysis of Fig. 2a, over the frequency band in which the impedance network IQ of the first stage is of uniform resistance, the effective overall transconductance of the system can be made very large and substantially independent of the nature of the final output impedance 23. Furthermore, the final output voltage is in phase with the input voltage while the output voltages of the first and second output circuits are 180 degrees out of phase, thereby providing a push-pull output with high amplification suitable for the audio-frequency amplification stage of a modulated signal receiver.

In Fig. 2a is shown an electrical equivalent of the circuit of Fig. 2 which is the same as that of Fig. 1a except that load 23 comprising impedance Z is incorporated in the second output circuit. This impedance Z has a negligible effect on the operation of the first part of the circuit since the transadmittance between the anode l6 and the first three electrodes is made negligible by suitably constructing and operating the screen grid 24.

The gain of the amplifier of Figs. 2 and 2a is:

' E,, I,,Z

and

I ==-g12, 16E1-(Q13, 1s+g1s, 16) E2 (9) where the symbols for transconductance are as explained above, and the symbols for the other quantities areas indicated on Fig. 2a. Using Equation 5 to express E2 in terms of E1 and substituting this expression for E2 in Equation 9, we get:

From Equation 5 it is seen thatvoltages E1 and E2 are in phase opposition and. might nullify each other in their influence on the second anode ourrent. However, the influence of E2 is made to predominate. That is to say, the second term in the bracket of Equation 11 is made much greater than the first by suitable choice of the value of resistance R and control of the values of the various transconductances by suitable tube design and operating potentials. Measurements on existing tubes indicate that gm. 1a is always small and, when a virtual cathode exists, 912,16 is usually small compared with the second term within the bracket of Equation 11. Thus Equation 11 may be written in approximate form:

the same as the grid-anode transconductance of r a conventional amplifier tube having the same.

cathode area and fineness of electrode structures. It is thus apparent that a considerable increase in effective transconductance is obtainable from this amplifier circuit if Q15, 13R: can be made to approach unity. This is not difllcult for low-frequency amplifiers nor is it difiicult within limits for tunable narrow-band radio-frequency amplifiers when there is substituted for resistor R an impedance network is which has a uniform resistance over the frequencies to which the network is tunable.

Since the second anode voltage Ep and the first anode voltage E: are 180 degrees out of phase, the

circuit of Fig. 2 may be used to obtain push-pull output with high gain. To make voltage E;. and

voltage E: equal in magnitude, the value'of im-. pedance Z should be chosen so that the gain as obtained from Equation 11 is theme as that obtained from Equation 5.. In the arrangement just described, the impedance network I! and the virtual cathode formed between electrodes I3 and I5 comprise means including the impedance network for maintaining the voltage of the output circuit of the anode IS in phme with the voltage of' the input circuit comprising input-circuit terminals H, H and 180 degrees out of phase with the voltage of the output circuit of the anode ll. By way of example, the circuit of Fig. 2 has been found to operate satisfactorily as a cascade amplifier with the following constants:

gm, 1a=1800 micromhos 91:, 1o= micromhos m, a=1650 micromhos F R1=1l20 ohms (input resistance of network 19) R2=100,000 ohms (resistance of resistor 20) The effective value of the first anode resistance R for this circuit is the parallel combination of the 1120-ohm and 100,000-ohm resistances and the self-resistance (1/913) of elect-rode l3 and calculates to be 987 ohms. This makes 015, BR equal to 0.788, which-is a value sufiiciently less than unity, to give fair stability yet large enough to give a satisfactory gain. Substituting the foregoing values in the bracketed part of Equation 11 indicates-that an effective transconductance between the first control electrode and the second anode of 14,400 micromhos is obtained.

For high-frequency operation of the amplifiers of Figs. 1 and 2, it is highly desirable that the vacuum tube ill have satisfactory interelectrode shielding. One structure suitable for high-frequency use is shown schematically in cross-section in Fig. 3 in which cathode ll hasbeamforming bailies 25, 25 on-eitherside thereof, while control electrode I2 is a conventional type of control grid. Anode i3 preferably is formed of spaced arcuate members of nonpermeable mate rial adapted to collect all electrons returned from the. virtual cathode formed just inside of c... itrol' electrode i5, thereby to prevent such electrons from beingreturned to and dissipated at screen grid ll. Thus, the screen grid H, which is disposed between control electrode l2 and anode l3, inhibits feedback to the input circuit in the amplifiers of both Figs. 1 and 2. The control electrode is preierably is of small pitch and wire size having supporting rods 26, 26 located on the axis of the beam in order to impart the correct-trajectories to the electrons, some of which are turned back, as indicated by the dotted lines. The screen grid 28 between control electrode I5 and anode i6 is provided for a conventional purpose and is of the usual screen construction, while the anode it consists of two flat opposed plates, such as those used in the type 6AB7 and 6AC7 vacuum tubes, to reduce output capacitance.

While there have been described what are at present considered to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from theinvention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. An amplifier comprising, a vacuum tube'having at least a cathode, a first control electrode, a first anode, a second control electrode, and a second anode, an anode circuit coupled to said second anode and said cathode, an input circuit coupled to said cathode and said first control electrode, an output circuit coupled to said cath ode and said first anode and including a two-- the dynamic first anode-cathode self-resistance comprising means providing a negative transconductance between said second control electrode and said first anode.

2. An amplifier comprising, a vacuum tube having at least a cathode, a first control electrode, a first anode, a second control electrode, and a second anode, an anode circuit coupled to said second anode and said cathode, an input circuit coupled to said cathode and said first control electrode, an output circuit coupled to said cathode and said first anode and including a twoterminal impedance network having a substantially uniform resistance over a desired frequency band, means for deriving a potential from said first anode and for applying it with the same polarity to said second control electrode, and meansior reflecting a negative resistance across said output circuit which tends to cancel the 3. An amplifier comprising, a vacuum tube having at least a cathode, a first controlelecv trade, a first anode, a second control electrode,

and a second anode, an anode circuit coupled to said second anode and said cathode, an input circuit coupled to said cathode and said first control electrode, an output circuit coupled to said cathode and said first anode and including a twoiii terminal impedance network having afsubstantially uniform resistance over a desired frequency band, and means for reflecting a negative resistance across said output circuit which tends to cancel the resistance thereof and the dynamic first anode-cathode self-resistance comprising means providing a negative transconductance between said second control electrode and said first anode of such value that the product of said transconductance and said resistance of said network approaches but is less than unity.

d. An amplifier comprising, a vacuum tube having at least a cathode, a first control electrode, a first anode, a second control electrode. and a second anode, an input circuit coupled to said cathode and said first control electrode, a

first output circuit coupled to said cathode and c said first anode and including a two-terminal impedance network having a substantially uniiorm resistance over a desired frequency band. an anode circuit coupled to said cathode and said second anode, means coupling said second control electrode to said first anode without substantial phase shift, and means for reflecting a negative resistance across said first output circuit which tends to cancel the resistance thereof and the dynamic first anode-cathode self-resistance comprising means providing a negative transconductance between said second control electrode and saidflrst anode.

5. an amplifier comprising, a vacuum tube having at least a cathode, a first control electrode, a first anode, a second control electrode, and a second anode, an input circuit coupled to said cathode and said first control electrode, a first output circuit coupled to said cathode and said first anode and including a two-terminal impedance network having a substantially uniiorm resistance over a desired frequency band, a second output circuit coupled to said cathode and said second anode, means for reflecting a negative resistance across said first output circuit which tends to cancel the actual resistance thereof and the dynamic first anode-cathode sell-resistance comprising means coupling said second control electrode to said'first anode without substantial phase shift, and means including said network for maintaining the voltage of said second output circuit in phase with the voltage of said input circuit and 180 degrees out of phase with the voltage of said first output circuit.

6. An. amplifier comprising, a vacuum tube having at least a, cathode, a first control electrode, a first anode, a second control electrode,

and a second anode, an input circuit coupled to said cathode andsaid first control electrode, a first output circuit coupled to said cathode and said first anode and including a two-terminal impedance network having a substantiallyuniform resistance over a desired frequency band, a second output circuit coupled to said cathode and said second anode, means for reflecting a negative resistance across said first output cir-' cuit which tends to cancel the actual resistance thereof and the dynamic first anode-cathode self-resistance comprising means coupling said second control electrode to said first anode without substantial phase shift; and means including said network for maintaining the voltage of said second output circuit in phase with the voltage of said input circuit and 180 degrees out of phase with the voltage of said first output circuit, the effective resistance of said first output circuit being soprpportioned with respect to the impedance of said second output circuit that the voltage of said first output circuit substantially equals the voltage of said second output circuit.

7. A regenerative amplifier comprising, a vacuum tube having at least a cathode, a first control electrode, a first anode, a second control electrode, and a second anode, an anode circuit coupled to said cathode and said second anode, an input circuit coupled to said cathode and said first control electrode, an output circuit coupled to said cathode and said first anode and including a two-terminal impedance network having a substantially unif-orinresistance over a desired frequency band, and means for obtaining regeneration comprising means for developing a negative transconductance between said second control electrode and said first anode, and means coupling said second control electrode to said first anode without substantial phase shift.

ROBERT L. FREEMAN. 

